It starts off very simple. An extra Captain and an AP at possibly inconvenient moment, but if the players think ahead and plan for it, it'll be easy to deal with.
It's an intentionally quick Chapter Quest designed to fit in easily and not disrupt the flow of a Story or Act. (I had that goal because I wrote these before the new FAQ came out that changed the official stance on Chapter Quests inside a Story or Act. They used to be mandatory, now they're optional.)
The first Quest links to two different Chains, but they both lead directly to the same final Chapter. If you play the game a lot, Chapter Quests will be reused, so I wanted to give the players one little decision that would let them switch it up over multiple plays.
As mentioned in a recent Myth-related post, lairs that are tucked away in hard-to-access areas can slow the game down quite a bit, so the Lairs Beneath quest should be used with caution if your Heroes don't have good equipment or a solid plan for dealing with a high-TN Lair.
Fallen Beneath is probably easier overall, but features a unique "one-way progression" mechanic that threatens to separate the Heroes, so difficulty is variable depending on the size and status of the Realm Tiles involved. If you leave an active Lair behind, you'll almost certainly regret it.
The final chapter is very simple. It's a half of a fight with the T1KL. Sort of a warm-up to eventually tackling the Boss in earnest in some Story Quest. The "Boss Stages" are an interesting mechanic, and just doing Stage One was a simple and elegant way to get more use out of the boss without adding too harsh a difficulty spike if this card came up in the middle of one of the more challenging Acts or Story.
The consequence for abandoning the Tile is meant to represent that if you don't quickly beat the monster into submission, it and its brood will remain active in the region. It assumes that the default practice is to remove completed Quests from the deck, but that's not very clear in the rules and some groups may not play that way.
Possible Revisions:These Quests are still pretty rough at this point, and may see some revision a little bit down the road. I see now how Mercs/Megacon made so many quests with ambiguities on them, as the limited space on the cards makes it very likely that your first draft will leave out something important. Editing with fresh eyes is pretty important.
Fallen Beneath may be tweaked to give the Hero sucked down the hole an immediate Treasure Draw. IIRC that was my intent when I made these a week and a half ago, but that timing isn't even hinted at on the card.
Lairs Beneath may need to be dialed back a bit in light of our recent difficulties with a hard-to-reach lair in normal play. Also, I envisioned the holes as 1x1 in size, and when I was making the card I hadn't yet realized there was a 2x2 hole token in the main set, so it may need extra text or a custom token to clarify that.
I may also reduce the Serendipity reward on the first quest to a single point (since it's pretty easy to beat), but then add another 2-point serendipity reward to one of the second-link quests instead of the current Treasure Bag manipulation. Rewards are the trickiest part of Quest design in Myth. Coming up with interesting scenarios and special rules is much easier than finding innovative new rewards. Assuming the Token count in the main box are intended as limits, your Treasure Bag ends up maxed-out long before you complete all the Bag-Manipulation Quests in the main game. I should probably brainstorm a bit before I do the revisions here or make more Quests.
There's also a temptation to expand the chain out a bit. I have this idea for turning the Aracnos from Galaxy Defenders into a Myth Captain or Mini-Boss, and I think I would prefer to shoehorn that into this Chain rather than build another Crawler-themed Quest Chain to showcase it.